Circuit for adjusting symmetrical band-pass systems



CIRCUIT FOR ADJUSTING SYMMETRICAL BAND-PASS SYSTEMS Filed Aug; 27, 1956 R. E. MOREN July l2, 1960 2 Sheets-Sheet 1 A rroRAEv 10...(JJ-Ow0 526.38 tz: www ,mwlw l M5055 ...www Etam NQ H waoomo. omo l mohouo mnoru NT Nam mw U (w States Patent 2,945,177, Patented July 12 1960 'GIRGUIT FOR ADIJUSTING SYMMETRICL nativi):Press'.SYSTEMS..V Robert. E..M oren,Graham,. N.'C`., assignor to Western. Electric Company, Incorporated, New York, NZY., a.

'corporation ofNw York` l Filed Aug.:zr/,wssgsennrot606,362.

`1. Claim.. (ClQ.324-'57).

The. presentzinventiont relates -to methodsof: `and ap,- paratusfon nesting-V and adjusting. electrical equipment particularly; ofthe type, utilizing-asymmetrical band-pass. system.`

Anobject'ofthe presentinventionisrto.providemethods, and apparatus whiohfsimplifyfand. expeditethetesting of. the gain versus frequency characteristicfof a 4unitem: ploying a band-pass system, such as an intermediate frequency arnpliien,

A feature of the invention lies in methods of and apparatus for testing? -at-unit o'tlie'bandpa'ss: type for symmenny of response at -two distinct frequencies equally displaced on:` opposite sides of the central frequency in theafoperatingrband of the unit. l

-An advantage of the present testing. methods: andz appartatusf resides in its adaptability for-` use" in testingother gain versus 'frequency characteristics of a unit having a band-pass system addition to@ its :response at two separated poirltsfinthe operating band; A

t lit iserhef usualil practice. in testing amplier units` of the type mentioned to obtain a response curveY overaband of frequencies by employing a signal source which is swept over the operating frequency of the unit, the output being detected and displayed on a cathode-ray oscilloscope. This system requires ,that the oscilloscope include precisely positioned markers in addition to requiring frequent gain measurements to insure accurate calibration of the oscilloscope. It is particularly diicult to arrange well-defined markers on the oscilloscope for readings at the lower radio frequencies. According to the present invention the aforesaid disadvantages are eliminated, since the symmetry of response may be determined by use of an oscilloscope without markers.

A clear understanding of the invention may be obtained by a reading of the ensuing description in conjunction with the drawings wherein:

Fig. 1 is a schematic diagram of an illustrative system embodying :the invention;

Figs. 2 and 3 illustrate wave shapes appearing at different points of the system during the testing procedure, and

Fig. 4 illustrates lthe gain versus frequency characteristic of a unit under test.

As illustrated in Fig. 1, an apparatus embody-ing the present invention includes a pair of crystal controlled oscillators 10 and 11, each of which may comp-rise a number of crystals designed to operate at different -frequencies in lthe operating range of the units to be tested. During a given test one crystal oscillator 10 provides a frequency fr displaced yfrom the central frequency fc of the unit under test, while -the other crystal oscillator 11 provides a frequency f2 equally displaced but at the opposite side of the central frequency. While crystal oscillators are specified, other sharply tuned and stable oscillators may be substituted therefor.

The outputs of the respective oscillators 10 and 11 are fed to amplifiers 12 and 13, which are alternately cutoff and rendered conducting `by means of a square wave generator `14. The square wave outputs of the generator 14'are yslown in lthe upper two curves offFig'. 2, the outputs4 representative, alternately, of. frequencies, fi. andf fz. rer

spectively, .bel'ow andab'ove. thfecentral frequency appears.: across the potentiometer 16..

Tliesignal's appearingI at thepotentiometer 1'6 are in uirnV passed through.' a bulr ampl'ier 1T, through the` unitV 1.8 under test. to. a catl'iodefollower 19, used. for inf ped'arlce. matchingpputposes, to. a-.d'etector 21 whose., outfy put may be connected' toa, voltmeter 22,. preferably a.. vacuumA tube.- volt-meter,A or, to. a. cathode-ray. Oscilloscopev Zrequring, no special.` markers.. The detector 21'. pro;- dices at its. outputtlie envelope of'tlie-combinedaliternate. of'. frequencies, fyand; f2,as sho-wn. by curveA D in 1g. i i

Priorgto makingaftest, for symmetry, it is desirable to', check.. thee signal appearing across the balance p0.` tentiometer: 16. at tlieA outputs, of, the amplifiers 1'Z` and 13T to.t determine wlietlier the. alternate frequenci'. fil. andi fiare. oequalamplitude.. For purposefthe test unit. I'Sis. bypassedbymovlng, a. switch 2li-4tov its upper position while switch 26. moved to its closed position.k The rectified' ern/.clope from the detector Ztl? is. now. displayed on. theolscilloseope 23,' While. the balance potentiometer4 1li'` is set. to a. position Whereat the` rectiiief envelope." indicates. equaL signals. at the. fre.quericis` fi 4and fg, as@ sliov'vnbythecurveB. ofllii'g,` 3,;

Assume, for example, that it is desired to test' an intermediary frequency amplifier, which is designed for a center frequency of 4.3 mc. and is required to have a maximum gain of a predetermined magnitude at the center frequency and a response curve such that the output frequencies 30 kc. on each side of 4.3 mc. are a certain number of decibels down in gain from the maximum gain, and more particularly, for the test being considered, that the gain be equal at such points on the curve. With such requirements the crystal oscillators 10 and 1-1 provide, respectively, frequencies f1 and f2 of 42,70 kc. and 4330 kc. After the balance potentiometer 16 has been adjusted as described above, switches 24 and 25 are moved to ltheir lower positions. The oscilloscope is now read and, if the conditions of non-symmetry illustrated by curves A and C of Fig. 3 prevail, the unit 18 under test is adjusted until the response is symmetrical, yas indicated by curve B of Fig. 3. The invention thus provides a quick and accurate indication of la non-symmetrical response of a unit under test so that the latter may be adjusted readily to rectify such condition. In some instances it may be desirable to test units in the manner described at two or more points asymmetrically related in the operating band.

If further and more elaborate tests of frequency versus gain characteristics and adjustments of a unit are required, the system includes, as shown in Fig. l, a variable frequency oscillator 237, and an eye .tube 28 for checking the accuracy of this oscillator against one of the crystal oscillators 10 and 11, for example, the oscillator 10, as shown in Fig. 1. The Variable frequency oscillator output is passed to a limiting amplier 29, which assures an output of uniform magnitude for application to the buffer amplier 17. The variable frequency oscillator Z7 is set to the center frequency 4.3 m.c., which may be shown by a reading on either the voltmeter 22 or the cathode-ray oscilloscope 23. Based on such readings, the

unit `under test may be adjusted until the desired maximum response is attained. It is, of course, 'to be under- Y stood that suitable switches'and control knobs may be provided in the-testing equipment to facilitate the testing and adjusting procedures. Y

Inthe chosen example of an amplifier to be tested, the response curve characteristic illustrated in Fig. 4 may be checked by simply tuning the variable frequency oscillator from 4.3 rnc., to 4.33 me., noting the level change on the voltmeter 22, and then tuning the -variable f-requency oscillator (from 4.3 mc.) -to 4.27 rnc., noting the level changes on the voltmeter which, of course, may be calibrated to provide a direct reading in decibels. It is only necessary to occasionally make a check, and adjnstment, if necessary, of the voltmeter 22 rto assure that accurate readings are being obtained. In Vcarrying out' the tests for maximum responses at the cent-ral frequency, andin determining the response curve, it is possible to use instead of variable frequency oscillators, the crystal' oscillators and 11, since each may include several crystals operating -at ditferent frequencies, as mentioned in an earlierpart of specification.

It is to be understood that the above-,described arrangements are illustrative of the invention. Other arrangements may be devised by those skilled inthe art without departing from the spirit and scope of the invention.

What is claimed is:

Av testing apparatus for visually indicating on an oscil- `los'cope the frequency 4response characteristics of an amplifier unit designed to operate about a central frequency, which comprises two oscillators for applying input signais, one of the oscillators being stable and sharply tuned at a frequency that is a predetermined number of cycles below the central frequency, and the other oscillator being stable and sharply tuned at a frequency that is said predetermined number of cycles above the central fre- Y quency, two amplifiers, means for applying said input signals to said two amplifiers, a Squareswave generator connected to each of the ampliiers for alternately rendering the amplifiers conductive and non-conductive, one of the amplifiers being in Ithe conductive condition when the other amplifier is in the non-conductive condition, a balance potentiometer including two terminals, one of the potentiometer terminals connected to an output terminal of one of the amplifiers and the other potentiometer terminal connected to an output terminal of the other amplifier, a detecting Icircuit coupled to the output of the amplifier unit to betested for observing simultaneously the response of Athe amplifier unit to the two signals from the oscillators, said detecting circuit including a detector' and the oscilloscope, and an Iadjustable tapping point on the potentiometer coupled -to the input of the ampliier unit for simultaneously adjusting the relative magnitudes of the outputs of the amplifiers to simultaneously vary said outputs until they obtain an initial condition displayed on Ithe oscilloscope by a series of visual flattop envelopes of equal shapes and magnitudes, the topsof said envelopes defining a straight line, said-envelopes remaining in vthe initial condition -if the amplifier unit responds equally with respect to both of said frequency signals and if said amplifier unit does not respond equally with respect to both of said frequency signals, said tops of adjacent envelopes varying in height to define on the oscilloscope a Series of broken, staggered lines.

References vCited in the le of this patent UNITED STATES PATENTS OTHER .REFERENCES v Ring: Meas. of Delay Detortion in Microwave Repeaters, Bell 'System Tech. Journal, vol. 27, April 1948, pp. 247-264. 

